A variety of moisture absorbing desiccants have been developed from materials ranging from inorganic salts to zeolites. Desiccants have also been incorporated into polymers, in an attempt to create materials that absorb moisture. There is a need, however, for materials that either absorb or control the permeation of moisture or other chemical species that are rugged, do not generate contamination, and are reusable.
Polyimides are a class of thermally and chemically stable polymers used in a variety of applications. The most well known commercially available series of polyimides are sold by DuPont™ and marketed under the trade name Kapton®. There are currently sixteen different types of Kapton® listed on the DuPont™ website. They all contain a polyimide framework but differ by their chemical or physical properties and are targeted for specific applications.
Certain polymers such as polyimides are known to absorb a small amount of water at ambient to high levels of humidity (<5%), however, they are not employed for that purpose. The use of a polymer to aid in moisture contamination has not been realized. Moisture contamination has been a general concern for many aerospace materials and systems including optical systems. Moisture as a contaminant is a common problem for space hardware, especially sensors that operate in the cryogenic temperature regime where ice formation is a possibility. Typical mitigation strategies involve the use of active thermal systems to “bake” off the water. The approach assumes a certain amount of moisture will be absorbed in porous materials, such as composite structures, thermal blankets, and other non-metallic components. While removing water on-orbit has proven effective, it is time consuming and requires intervention from a ground operator. The ability of a space grade material to sense and absorb water would be of great use, possibly eliminating or reducing the need for on-orbit bake-outs.
The synthesis of polyimides is a well-known process. By way of example, for poly(4,4′-oxydiphenylene-pyromellitimide) (Kapton® H), the synthesis begins with the condensation of a diamine with a dianhydride to form poly(amic acid). The poly(amic acid) undergoes imidization upon heating to form a polyimide. The materials are commonly cast in thin layers before heating to form thin films. Other types of polyimides can easily be prepared by making substitutions on the two building blocks (diamine and dianhydride).
Many methods for modifying polyimides have been developed. A vast amount of effort has been applied to modify well known polyimides both before and after the imidization step (i.e., heating).
It would be useful to be able to provide a moisture interactive material (i.e., moisture sensing, absorbing, permeating, reacting, detecting, capturing, and/or releasing) that has one or more of the following characteristics: excellent thermal stability, solvent resistance, good mechanical properties, radiation resistance, low thermal expansion, and wear resistance. In particular, it would be useful to be able to provide a moisture interactive material that has improved moisture absorption capacity and provides an indication of an amount of moisture presently absorbed by the material, while still retaining good mechanical properties. It would also be helpful to be able to provide an easily implemented process for synthesizing such moisture interactive materials.